Stirling cycle machines, whether in the form of engines (driven by an external heat source) or heat pumps (driven by an external source of mechanical power), typically utilize three forms of heat exchangers associated with an oscillating displacer. The displacer reciprocates between an expansion space and a compression space within an enclosing cylinder in a Stirling cycle mode of operation.
The three "heat exchangers" are typically termed a "heat acceptor" or heater, a "heat rejector" or cooler and a "regenerator". The general design of these elements is well-known in this technological field. It is summarized in U.S. Pat. No. 4,671,064, issued June 9, 1987 to Maurice A. White and Stuart G. Emigh, which is hereby incorporated into the present disclosure by reference.
The present invention arose from a desire to simplify the machine structure and to improve the operational efficiency of two cylinder opposed Stirling cycle machines by reducing the resistance to flow and/or increasing heat transfer of working gas between the expansion and compression spaces. This is accomplished by interposing a common heat acceptor between the coaxially aligned movable displacers. Among the design criteria for such a heater head are high heat transfer efficiency, reduced fluid flow resistance, ability to operate under conditions of high temperature and pressure, and low cost of construction.
To minimize the physical volume of such machines, annular regenerators and heat exchangers are often arranged about the exterior cylinder walls. However, the heat acceptor, within which substantial quantities of heat must be exchanged with an external source, usually requires inclusion of elongated tubes or other heat transfer passages in which the working fluid of the machine must effect a complete reversal of direction--a 180.degree. bend in the fluid path. This is illustrated schematically in FIG. 1.
FIG. 1 schematically shows a Stirling cycle machine having an oscillating displacer 11 contained within a surrounding cylinder 10. A conventional clearance seal 12 is illustrated between cylinder 10 and displacer 11 to separate working fluid (typically a gas) in the expansion space 13 and compression space 14 at the respective ends of cylinder 10.
The three heat exchangers associated with the working fluid are shown as a heat acceptor 15, a heat rejector 17 and an annular regenerator 19. They are arranged between the expansion space 13 and compression space 14 in the conventional order associated with Stirling machines.
The prior art heat acceptors 15 (FIG. 1) typically include a heat supply connection shown at 16. The heat supply connection 16 is operably connected to an external heat source (not shown). In the case of a Stirling engine, the heat source might be a source of combustion, solar energy, radioisotope, etc. In the case of a Stirling heat pump, the heat source might be a building, electronic equipment, or other external equipment (not shown) that requires heat removal. In the schematically illustrated device, heated gas or vapor is directed into the heat acceptor 15 through a conduit 16, as shown by arrow A, and condensed liquid will be returned along the walls of conduit 16, as shown by arrows B. The heat rejector 17 is provided with heat sink connections 18 through which fluid (gas or liquid) flows to carry heat from the machine, as shown by arrows C.
It is typical in Stirling cycle machines to direct working fluid through the various heat exchangers between the expansion space and compression space of each cylinder. In some two cylinder opposed Stirling cycle machines, there has been proposed branching of working gas from a common plenum or expansion space between the two displacers. However, this requires separate flow paths through two heaters, whether arranged annularly about the coaxial cylinders or in a parallel fluid path alongside them. The open plenum typically presents a relatively large volume of working fluid that is non-processed between the two opposed displacers. It is essentially "dead volume" that does not contribute to operational efficiency in the machine.
The current invention streamlines the flow of working fluid through a common heat transfer head, providing two elongated fluid paths in the heat transfer head for effective heat transfer from a common heat source. It eliminates the 180.degree. fluid path bend and reversal of fluid direction that have been associated with prior heat transfer heads at one axial end of a cylinder in a Stirling cycle machine. It does this by providing cross flow of working fluid between the expansion space of each coaxially aligned cylinder and the compression space of the other. Further details will be discussed with respect to the specific physical embodiment of the invention illustrated in the accompanying drawings.